When considering understanding the contact state of a contact surface using a tactile sensor, there are vectors of three components representing magnitude and direction of force acting at each point of the contact surface. This is represented as f(x,y) in the coordinate system of FIG. 1. Here, f is a vector, and so actually has three components x, y and z at each point. When explicitly expressing each component, it is represented as f(x,y)=[fx(x,y), fy(x,y), fz(x,y)].
Some of inventors of the present invention et al. have proposed an optical tactile sensor that is capable of measuring three-dimensional force vector distribution. The optical tactile sensor is disclosed in WO02/188923 A1 and incorporated herein by reference. A principle of the optical tactile sensor will be explained based on FIG. 2. The optical tactile sensor comprises a transparent elastic body and a CCD camera. By photographing spherical markers embedded in the transparent elastic body by the CCD camera, internal strain information of the elastic body is measured when a force is applied on the surface of the elastic body, and force vector distribution is reconstructed from the information.
By taking an image of the spherical markers by a CCD camera from z-direction where an elastic body surface is taken as the x-y plane and an orthogonal direction to the x-y plane is taken as the z-axis, movement of a point to be measured when force is applied is measured as a movement vector in the x-y plane. However, it is difficult to reconstruct the force vector distribution from the strain information because an amount of information is insufficient. Therefore, N×N red spherical markers and blue spherical markers are arranged at different depths in the elastic body as points to be measured to obtain two sets of two-dimensional movement vectors with different depths as two pieces of different information, thereby increasing the amount of information to reconstruct the force vector distribution.
According to the above-mentioned optical tactile sensor, the optical tactile sensor having a flat surface is generally employed. Since the surface is photographed as two-dimensional image information, application of flat surface that corresponds to the two-dimensional image information may be a natural choice. Also, in case of a sensor with a flat surface, it is easier to reconstruct force vector distribution.
This type of optical tactile sensor has advantages in that it can measure three-dimensional force vector distribution and has an elastic body providing a flexible surface to be contacted by an object. For example, in a situation where the optical tactile sensor is provided at a robot hand of a humanoid, it is necessary to hold a glass without breaking and dropping. To prevent the glass from dropping, it is necessary to sense a force acting in the direction parallel to the surface of the glass. This is possible with the above-mentioned optical tactile sensor. Here, when considering applications of this type of optical tactile sensor for various purposes, it is necessary to construct a tactile sensor with an arbitrary curved surface not with a flat surface. However, it is difficult to reconstruct force vector distribution with an arbitrary curved surface. In this regard, a tactile sensor with an arbitrary curved surface is disclosed in “Development of arbitrary curved type tactile sensor using pressure conductive rubber”, Shiinojo et al., Robotics Society of Japan, 1 G24, 2002. However, it is not possible to acquire force vector distribution by this sensor.
An object of the present invention is to provide an optical tactile sensor with an arbitrary curved surface.
Another object of the present invention is to reconstruct force vector distribution applied to an arbitrary curved surface from marker information.
Still another object of the present invention is to provide an optical tactile sensor that is capable of being used as a tactile sensor for a robot hand or a computer interface.
Still further object of the present invention is to provide a method of obtaining a transfer function by which a force vector distribution is calculated by using marker information.